1. Field of the Invention
The present invention relates to an electrostatic spray painting apparatus having an intermediate reservoir incorporated therein for temporarily storing therein conductive paint to which a high voltage is directly applied.
2. Description of the Related Art
As one mode, i.e., one functioning arrangement employed in an electrostatic spray painting apparatus, there has heretofore been provided an intermediate reservoir between a spray gun and a paint feed source.
The intermediate reservoir is normally provided with a storage chamber having a given capacity. In addition, the intermediate reservoir is constructed in such a manner that a predetermined quantity of paint can be supplied to a spray gun by a piston reciprocatively movable within the storage chamber.
Many attempts and improvements have been made with a view toward avoiding the mixture of paint prior to color changeover or replacement with the new paint upon the use of the new paint owing to the fact that the previously-used paint remains in the intermediate reservoir and adheres thereto. As one typical example, there can be mentioned a technical idea disclosed in each of U.S. Pat. No. 4,771,729 entitled "SYSTEM FOR AUTOMATIC ELECTROSTATIC SPRAY COATING" and Japanese Patent Application Laid-Open No. 63-310671 entitled "METHOD OF SUCCESSIVELY SPRAY-COATING WORK WITH CONDUCTIVE PAINTING MATERIAL".
According to the former disclosure, however, cleaning liquid is blown toward only an end face of a piston when it is desired to clean the inside of the intermediate reservoir at the time of the color replacement. Therefore, paint prior to the color replacement is not fully removed from a peripheral edge of the piston end face, to which the cleaning liquid is not fully applied.
According to the latter disclosure, the quantity of cleaning liquid flowing in the intermediate reservoir is not sufficient to clean the inside of the intermediate reservoir at the time of the color replacement, with the result that the removal of paint from a piston end face is substantially insufficient.
Even in either case of such two prior disclosures, O-rings are used as sealing members provided between the piston and the cylinder. Therefore, the paint, which has been injected into a cylinder chamber may slightly leak at the time that the paint is fed according to the displacement of the piston.
As a result, some paint remains in an inner peripheral wall of the cylinder chamber and adheres thereto, so that such paint mixes with the new paint, thereby causing the inconvenience that paint of an unadulterated color cannot actually be applied to a workpiece.
In order to overcome the foregoing inconvenience, the piston should be cleaned when withdrawn, thereby causing the problem that the cleaning time of the inside of the intermediate reservoir is time consuming and a large quantity of cleaning liquid is required.
Now, there are situations in which a relatively small quantity of paint may simply be supplied to a spray gun owing mainly to the shape and the dimensions of a portion to which paint is to be applied. It has, however, been pointed out as a problem that even excess paint is stored in a storage chamber of a normally-used intermediate reservoir, resulting in an inefficient cleaning process. Therefore, there has been proposed an apparatus having means for measuring the quantity of paint supplied to an intermediate reservoir, and means for confirming whether or not a predetermined quantity of paint is supplied to the intermediate reservoir. However, this apparatus has the problem that it is complex in structure and expensive as a whole, for example.
On the other hand, there is known a color-of-paint changeover system disclosed in Japanese Patent Application Laid-Open No. 2-2885, for example, as a technique for applying a high voltage to conductive paint so as to subject an object or workpiece to be coated with the paint to electrostatic spray painting or coating.
According to this disclosure, the conductive paint is first introduced via an insulated line into an intermediate reservoir from a paint feed source. Thereafter, the insulated line is cleaned and dried so as to be brought into an electrically-insulated state (voltage block), thereby preventing current from leaking to the paint feed source side. Under this condition, conductive paint to which a high voltage is applied is supplied to a spray gun from the intermediate reservoir, so that the electrostatic spray coating is transferred to the object or workpiece.
In this case, pressurized air has been supplied to the intermediate reservoir from a pressurized-air feed source in order to feed the conductive paint to the spray gun from the intermediate reservoir. After the delivery of the conductive paint to the spray gun is completed, the pressurized air is discharged to the outside from the intermediate reservoir through a vent portion.
In the conventional art referred to above, however, the pressurized air in the intermediate reservoir is adiabatically-expanded so as to be discharged to the outside from the vent portion. Therefore, a path extending from the intermediate reservoir to the vent portion is cooled, thereby condensing moisture or water in the atmosphere into dew. As a consequence, the dew is deposited on the path and the vent portion. Therefore, an undesired flow of electricity can easily occur owing to the deposition of the dew thereon when the high voltage is applied to the conductive paint in the intermediate reservoir.